Optical fibers provide communication bandwidths as large as tens of Terahertz (THz). Such large bandwidths are too large to be fully used by single-channel communication systems that transmit data by amplitude modulation of a narrow-bandwidth optical carrier such as by directly modulating a distributed feedback laser. Factors limiting the bandwidth occupied by simple amplitude-modulated systems include bandwidth limits on available electronic and optical components such as optical modulators and detectors, as well as fiber dispersion.
In order to more fully use the available bandwidth of an optical fiber, multiplexing techniques have been developed that permit several communication channels to use the same optical fiber. The total fiber bandwidth used by these channels can be as large as the sum of the individual channel bandwidths. Conventional methods for multiplexing in optical communication systems include time-division multiplexing (TDM), wavelength-division multiplexing (WDM), and optical-code-division multiple access (OCDMA). These optical multiplexing methods are typically used in long haul, point-to-point communication and in short haul, ring-topology communication systems. Each of these methods has benefits as well as drawbacks associated with it.
A passive optical network (PON) capable of transmitting and receiving data from a large number of users simultaneously reduces cost and increases system reliability. A PON includes passive optical components that route optically encoded data streams to and from designated users or network nodes. Communication systems based on the Synchronous Optical Network (“SONET”) are not PONs since each user or node in a SONET system must have active equipment to detect and demultiplex the data stream to recover data, and then retransmit data to the next user or node.
WDM systems are PONs, but each user of a WDM system is assigned a specific wavelength, so that WDM-based PONs are expensive and are not readily reconfigurable.
The use of OCDMA in a PON is advantageous in that data encoded onto a single transmitter can be distributed among many users because data is distinguished by a complex temporal code, and not wavelength as in WDM systems. Furthermore, CDMA coding has the advantage that the number of users is flexible, dependent on system usage. Also, most OCDMA implementations inherently offer hardware encryption by making channel decoding difficult without knowledge of the OCDMA encoding scheme. However, flexible tree-architecture PONs based on OCDMA typically require expensive equipment that must be located at each network node or an end-user location. Therefore, improved systems, methods, and architectures are needed for practical PON OCDMA systems.